1. Field of the Invention
The present invention relates to a ferroelectric/paraelectric multilayer thin film, a method of forming the same, and a high frequency variable device using the same, and more particularly, to a ferroelectric/paraelectric multilayer thin film composed of a barium-strontium-titanium thin film, a method of forming the same, and a high frequency variable device using the same.
2. Description of the Related Art
Recently, new services, such as high capacity next-generation wideband broadcast, communication, internet-combined mobile wireless multimedia system, ubiquitous communication, sensor system and the like, are realized into actualities. Therefore, development of new essential materials and components for wireless mobile/satellite communication and sensing systems having characteristics of a high speed, low power consumption and a low price enough to provide many services in various frequency bands is now being directed into importance. Therefore, it is highly demanded on technology development for ferroelectric thin film materials and devices, which are expected to complement the advantages and disadvantages of devices characteristics realized as existing semiconductors, micro-electro-mechanical systems (MEMS), magnetic substances, photonics, and to achieve excellent high frequency characteristics.
A high frequency frequency/phase variable device using such a ferroelectric thin film has characteristics of a high speed, low power consumption, small size, light weight, low price, high frequency/phase variable characteristics, broadband, system on a chip (SoC). However, the most significant problems in the development of the high frequency frequency/phase variable device using such a ferroelectric thin film are their high insertion losses, frequency/phase variable rate, high operation voltage and the like. Many efforts have been made to develop materials for ferroelectric epitaxial thin films having an excellent high frequency dielectric property and devices using the same in order to improve the characteristics of the high frequency ferroelectric variable devices. To this end, a ferroelectric epitaxial thin film material is required to have a high tuning rate of a dielectric constant and a small dielectric loss.
Among many ferroelectric materials, barium-strontium-titanate (Ba1-xSrxTiO3) (hereinafter, referred to as “BST”) is known as a thin film material good enough to realize a ferroelectric high frequency variable device because of its high tuning rate of a dielectric constant and a small dielectric loss. Further, many studies are being made to improve the device characteristics by improvement of dielectric properties, such as a high tuning rate of a dielectric constant and small dielectric loss of a BST thin film.
Up to now, many researches have been made on doping, a high growth temperature, defect compensation of a Ba/Sr ratio, thickness dependency, and the like to provide a BST thin film having a high dielectric constant tuning rate and small dielectric loss. However, there is limitation in achieving a BST thin film having properties comparable to dielectric properties of BST single crystal. A tuning rate of a dielectric constant and dielectric loss of a ferroelectric BST thin film grown on the single crystal oxide are known to be significantly influenced by oxygen vacancies, a thickness of a thin film, a size of crystalline particle, doping element, a Ba/Sr composition ratio, strain/stress inside a thin film, crystallinity of a thin film, thin film growth conditions such as temperature, oxygen partial pressure, growth rate, and the like. Particularly, epitaxial growth of a BST thin film is not easy at a low temperature due to large difference of lattice constants between the BST thin film and oxide single crystal substrate, and further, high strain/stress inside the thin film is known as a main reason to deteriorate a tuning rate of a dielectric constant and dielectric loss. Because of that, propagation loss of high frequency signals is increased, and a ferroelectric high frequency variable device having effective and excellent characteristics was difficult to realize.